Okay, so my sister has a Walmart bought CT and his fins were rather ratty and curled (I'm assuming because of our Texas water hardness, and age more than likely.) We're now using distilled water by the gallon, and it seems to be helping a bit. Does anyone else use distilled water? Is it better than tap for their fins?

Wondering because I'm thinking about purchasing a nice CT from AB, but I don't want to risk ruining his fins with our water. If I just use purely distilled, would that prevent curling?

Distilled water (or any form of purified water) is actually very bad for fish. It is completely devoid of minerals and buffering capacity. Because the mineral content is so low, it sucks the minerals from your fish. Because distilled water has no buffering capacity it can cause wild pH swings which are stressful to your fish. If you want to soften your water, you should be diluting your tap water with purified water and not just using purified water. Diluting your tap water with 50% distilled water will reduce the hardness by half. Diluting it with 75% distilled water will reduce the hardness by 75% and so forth.

As far as water hardness and crowntails, I've heard that hard water can cause curling, but I have no direct experience with it because I don't like crowntails and I have soft water.